The present invention relates generally to coherent radiation systems, and more particularly to a method of reducing noise introduced into systems using coherent radiation sources.
Numerous systems employ the use of coherent radiation sources to irradiate film, detectors, recording media, and the like. For example, the generation of holograms involves the recording of a diffraction pattern generated by the interference of two beams of coherent radiation, such as is provided by laser sources, or the like. The holographic exposure may be recorded on photographic plates, in a gelatinous substance, or in other available media as is well-known in the art. In order to record a particular holographic image, the coherent radiation source may be split into two or more beams to form the hologram.
An apodizer may be disposed along the optical path between the light source and the recording medium. An apodizer is an element which changes the distribution of the radiation passing through it. This apodizer may be either an aperture which limits the size or shape of the coherent beam, or may be a material which filters the beam in order to produce a more desirable beam energy profile, or the like. For example, one type of apodizer is disclosed in a publication entitled "Holographic filter that transforms a Gaussian into a uniform beam", by M. Quintanilla et al, Applied Optics, Vol. 20, NO. 5, Mar. 1, 1981. Another apodization process is disclosed in "Method for converting a Gaussian laser beam into a uniform beam," by W. Lee, Optics Communications, Vol. 6, No. 6, Mar. 15, 1981.
A major problem in producing holograms is that the optical components, including the apodizer, produce unwanted diffraction patterns which are recorded during the holographic exposure. For example, an aperture stop, such as a circular aperture, diffracts the light transmitted therethrough at the edge of the aperture. This diffraction of the light creates noise which is recorded during the holographic exposure. When the hologram is reconstructed, the unwanted diffraction noise is also reconstructed as an image of the aperture and reduces the quality of the desired image.
A variety of prior systems have been contemplated or used in order to reduce diffraction noise from coherent optical systems. For example, a photographic negative masking system is a standard commercial procedure for use with incoherent light, but introduces considerable diffraction noise with coherent optical systems. Grainless masking systems, such as photochromics, tend to be unstable and have phase variations which create noise in coherent optical systems. Filters may be ground out of colored glass which give even illumination in coherent light without diffraction noise. However, such filters affect the phase of the light wavefronts, and are therefore unacceptable for use in holography. Prior art coherent optical systems utilizing apertures have attempted to focus the defining aperture onto the holographic plane to eliminate the diffraction pattern. Unfortunately, optical constraints on many systems restrict the use of this method.
One particular method for use in imaging systems is disclosed in a NASA publication entitled "Elimination of Coherent Noise in a Coherent Light Imaging System", by Grebowsky et al. The publication is identified in a NASA Technical Brief and Technical Support Package number 71-10236, dated July 1971. Noise diffraction patterns created by dust, blemishes or scratches are blurred by spinning the lenses in the system about their optical axes. The diffraction patterns created by the dust, blemishes or scratches are blurred by the spinning lenses. The rotating lenses eliminate Newton's rings created in the image due to the dust and scratches.
Accordingly, it would be an improvement in the coherent optical systems art to provide for a method which reduces coherent noise recorded during an operation of such systems. In particular, it would be an improvement in the holographic art to provide a method which reduces unwanted diffraction noise during construction and reconstruction of optical holograms.